Compartmentalized micromagnet display device

ABSTRACT

A device adapted to provide a visual display under the influence of an exterior magnetic field which comprises a plurality of rotatable, multi-colored, non-magnetically interactive, permanently magnetized micromagnets disposed in a cavity within a body having a transparent surface and a plurality of relatively small compartments within the cavity. The device is illustrated by a sheet containing such micromagnets between the thin cavity of the front and rear surfaces of the sheet where the relatively even dispersion of the micromagnets in the sheet is maintained by such compartments whose barriers minimize displacement so that physical pressure exerted on the sheet, such as by a twist or flex, does not substantially alter the even dispersion of the micromagnets. 
     In an embodiment, the visual display device is a sheet, preferably a substantially transparent or invisible polymer sheet, containing relatively small compartments therein which have cellular and/or structural barriers which prevent substantial displacement therefrom by holding in such compartments rotatable micromagnets which are surrounded with a suspension or carrier fluid which allows such rotation. In the preferred embodiment the suspension or carrier fluid is thixotropic.

This application is a division of my co-pending Ser. No. 23,424 filedMar. 27, 1970 and now U.S. Pat. No. 3,938,263 which is acontinuation-in-part of Ser. No. 756,711 filed Aug. 6, 1968, nowabandoned, which in turn is a continuation-in-part of Ser. No. 553,086filed May 26, 1966, now abandoned.

This invention relates to improved magnetically actuatable visualdisplay devices and the methods for making same. More particularly itrelates to a device containing, in tiny cells or compartments, rotatablemulti-color micro-magnets which may be magnetically oriented to providea visual display. The dividing partitions between the cells orcompartments, preferably transparent and undiscernable to the unaidedeye, provide important structural contributions against physicalpressure on the device and in so doing prevent such pressure fromtranslationally displacing the micromagnets.

In U.S. Pat. No. 3,036,388, since reissued as U.S. Pat. Re. Nos. 25,363and 25,822, I have described color coded micromagnets which may beturned by an activating magnetic force to present Zones of one color oranother and which may be oriented selectively so that the colorspresented by some contrast with the colors presented by the others,producing thereby a visuably distinct pattern. The micromagnets are madeto possess a low volumetric magnetization so that their magneticstrength is insufficient to cause a magnetic interaction which wouldprevent selective orientation by an activation external magnetic force.Thus, the permanently magnetized micromagnets are non-magneticallyinteractive, i.e. they have a unit volumetric magnetization below thepoint at which the closely associated micromagnets tend to be attractedto each other in the system in which they are employed which will causethem to resist individual orientation by the external magnetic force ofthe writing instrument. They can be mixed with a liquid suspendingmedium and dispersed in the thin cavity formed between two spacedsurfaces, at least one of which is transparent. An inner face can beprovided with dimpled indentions which, while allowing axial rotation ofthe micromagnets, restrain their translational movement.

The micromagnets so suspended function quite satisfactorily, unless theshape of the thin cavity is changed by a physical distortion, such as atwist or a flex, which may displace some of the micromagnets, leavingportions of the sheet vacant and inoperable.

As will be apparent from the description herein, the problem oftranslational movement is prevented by the successful containment of themicromagnets in cells or compartments, the walls of which are invisibleor minimumly visible to the unaided eye. In one example, a continuoushardened transparent film, such as a synthetic polymer such as polyvinylacetate or copolymers thereof, etc., or a natural organic polymer suchas gelatin, etc. contains discrete fluid containing inclusions, therebeing mobile multicolored micromagnets entrapped within the inclusionsand being selectively orientable by application of magnetic fields tothe sheet, each of the said micromagnets being permanently magnetized toa constant magnetization vector; and the micromagnets having distinctzones of contrasting colors which are visible through such film.

In another embodiment, a substrate carries the micromagnets in asuspension liquid, the mixture having been first deposited in brokenpatterns on a substrate and then sealed into cells by a hardenablecoating, the vacant areas between the cells then having been filled withsimilar deposits, with these then having been so sealed.

And a third embodiment comprises a pre-constructed transparent carrierwith partitions so arranged that micro-magnets can be drawn intopre-constructed and pre-arranged inclusions.

In addition to the patent before mentioned, micromagnets of two or morecolors and of various shapes are disclosed in my U.S. Patent Nos.3,460,248 and 3,406,363 and any such micromagnets may be used in theinstant invention. For the purpose of illustration, herein, themicromagnets are shown with only two colors, though they may have more.For illustration, the micromagnets used in the continuous film are shownas platlet shaped and the micromagnets which were prearranged on acarrier, and the micromagnets which were injected into a preconstructedcompartmentalized carrier are shown, for illustration, as spherical orspheroidal.

The size of the micromagnets can vary widely. For example, themicromagnets useful in the practice of the invention are in a size rangesuch that they have a broad dimension between less than about 25 and1000 microns or greater. Micromagnets of about 45 microns provide asmooth uniform appearance at the viewing surface since the individualmicromagnets cannot be resolved by the eye. Micromagnets in the range ofabout 25 to 250 microns in size are preferred, but micromagnets up toabout 1000 microns are generally useful.

The micromagnets are carried in a suspending liquid in which they arefree to rotate upon the application of a magnetic field. It is desirableto provide this liquid with a viscosity and thixotropy such that acertain minimum force must be applied in order to rotate themicromagnets. Such viscosity and thixotropy provide a degree ofstability to the display device, minimizing unwanted disorientation ofthe micromagnets.

Other objects and advantages will be apparent from the accompanyingdetailed description and drawings wherein:

FIG. 1 is a diagrammatic magnified cutaway cross-section view of asubstrate coated with a micromagnet-containing coating of the presentinvention;

FIG. 2 is a greatly magnified cutaway cross-section view of a portion ofthe coated substrate of FIG. 1;

FIG. 3 is a greatly magnified perspective view of a platlet shapedtwo-colored magnetically orientable micromagnet.

FIGS. 4 and 5 are diagrammatic cutaway cross-sectional views of sheetswhere the micromagnets and suspension liquids are deposited and sealedin cells in broken or random patterns.

FIGS. 6 and 7 are diagrammatic cutaway cross-sectional views of sheetswhere the micromagnets and suspension liquids are deposited inpre-constructed and pre-arranged compartments.

FIG. 8 is a magnified side view of a spherical micromagnet.

When used in a display device, the micromagnet of FIG. 3 which can beemployed in FIGS. 1 and 2 as 30 will present first colored surface 12when the south pole of a magnet is passed over the display device, andsecond colored surface 14 when a north pole of a magnet is passed infront of the display device. Electromagnets, magnetic tapes, and othersources of magnetic fields can be used to actuate the display devices ofthis invention in addition to styli. FIGS. 1 and 2 show across-sectional side view of a substrate 20 with a magneticallyactuatable coating 21 of the present invention thereon which coating ismade up of a continuous phase 22 containing as a discontinuous phasetiny liquid-containing inclusions 24. The inclusions 24, on the average,contain one or more colored micromagnets 30 suspended within the liquid24 which are free to rotate therein when actuated by magnetic forces.Substrate 20 may be cloth, plastic, paperboard, metal, etc. or othersuitable backing. In a further embodiment, the coating 21 may be used asa free film without backing.

The magnetic orientation of the several micromagnets is illustrated bythe arrow in each of the FIGURES, the arrowhead in each instance, forconvenience, indicating the north pole.

The micromagnets shown in FIGS. 1, 2 and 3 are from sheets having twodistinctly colored layers, as will be described in the followngexamples. But as has been explained in the earlier patents mentioned,micromagnets possessing more than two distinct color zones, producedfrom sheets having more than two separately colored layers, can beoriented to intermediate degrees of rotation to produce combinations andblends of colors.

Coating 21 is formed by suspending micromagnets 30 in a suitable liquid24 which is preferably an oily liquid having a density and viscositysuch that the micromagnets will remain suspended therein without a greattendency to either sink or float. After thoroughly mixing themicromagnets in the liquid, this liquid with the micromagnets suspendedtherein may be mixed with a film-forming material in which it isimmiscible so that continued mixing will form an emulsion or dispersionof micromagnet-containing droplets within the film-former. This emulsionmay then be coated over the substrate 20 and hardened to provide acoating in which the micromagnet-containing inclusions form adiscontinuous phase within a continuous hardened film-former 22. Eachinclusion ideally should contain one or a small number of micromagnets.Film-former 22 may be a gelatinuous or waxy material or preferably anoil immiscible, hardenable plastic resin such as vinyl acetate,polyvinylpyrrolidone, ureaformaldehyde, polyvinylbutyral,polyvinylalcohol or methylcellulose.

The coating shown in the drawings contains a single layer ofmicromagnet-containing fluid inclusions in coating 21. A uniform singlelayer of this type is preferred for purposes of economy. It will beunderstood, however, that coating can be formed in which inclusions areseveral layers deep. Such multiple layers may be preferred where verycomplete coverage of the substrate is desired.

The following examples in which all proportions are given in parts byweight unless otherwise indicated, will serve to illustrate but notlimit the invention.

EXAMPLE 1

A hardenable white composition was prepared by mixing the followingingredients:

    ______________________________________                                                                  Parts                                               ______________________________________                                        Styrene-butadiene copolymer containing                                        65% by weight TiO.sub.2 pigment (Goodyear                                     Pliolite 1A-S5)             16.6                                              Toluol solvent              25                                                ______________________________________                                    

This composition was roll coated with a rotogravure 120 Tri-helicoidroll at a rate of 15 yards per minute onto a plastic release web. Thiscoating was oven dried at 250°F. The coating had a dry weight of 0.00073gm/cm² of area. A black colored hardenable coating composition wasprepared by mixing the following ingredients:

    ______________________________________                                        Styrene-butadiene copolymer containing                                        40% carbon black (Goodyear Pliolite 2C-S5)                                                                4.5                                               Barium ferrite containing polymer                                             (.224 parts barium ferrite, 0.071 parts                                       clear styrene-butadiene copolymer Pliolite                                    S5-E, 0.295 parts toluol)   0.59                                              Toluol                      14.5                                              ______________________________________                                    

This composition was coated using an 80 Tri-helicoid rotogravure rollover the white layer and oven dried at 250°F. The black layer had a dryweight of 0.00065 gm/cm². The combined layers had a weight of 0.00138gm/cm² and a calculated density of 1.77 gm/cm³. The combined layers hada barium ferrite content of 2.2%. The hardened material was passed onthe carrier web between the poles of an electromagnet, magnetized at9000 gauss at a speed of 1 foot per second. Material was removed fromthe carrier web by flexing and air blasting, and then was conveyed athigh velocity through a tortuous path where it impinged against itselfand other obstructions until the average particle diameter was about11/2 times its thickness. Outsized particles were removed by screening.

A suspension in oil of the black and white micromagnets was formed bymixing the particles into the following oily mixture:

    ______________________________________                                                                  Parts                                               ______________________________________                                        Low molecular weight chlorotrifluoroethylene                                  polymer having a density of 1.9 and a                                         Brookfield viscosity of 72°F., No. 1 spindle                           30 rpm, 124 centipoise (Kel F Oil No. 3, 3M Co.)                                                          300.00                                            Oil having a density of 0.85 and a Brookfield                                 viscosity of 24 (No. 1 spindle, 60 rpm, 72°F.)                         (Retrax, Std. Oil Co.)      269.00                                            Purified bentonite with an organic base,                                      gelling agent (Bentone 38, Nat. Lead Co.)                                                                 1.00                                              Stearic Acid                4.75                                              ______________________________________                                    

The oil mixture had an approximate density of 1.21 and approximateviscosity of 70 centipose when measured on the Brookfield Viscosimeterusing a No. 1 spindle at 30 rpm.

A resin mixture was formulated by mixing the following ingredients:

    ______________________________________                                        Copolymer of vinyl acetate and a carboxylated                                 monomer (Gelva C5 V10, Shawinigan Corp.)                                                                  86.0                                              Saturated polyester resin plasticizer (Harflex                                340, Harchem Div., Wallace & Tiernan Inc.)                                                                8.6                                               Methanol                    180.0                                             ______________________________________                                    

The resin mixture had a calculated density of approximately 0.91 and aBrookfield viscosity of 140 centipose. An oil resin emulsion was formedby mixing 1 part by volume of the magnet contained oil mixture with 3.5parts by volume of the resin. After mixing, an emulsion was formed inwhich the resin was a continuous phase having dispersed therein oildroplets averaging about 10 mils in diameter as a discontinuous phase.One or more colored micromagnets were contained within the preponderantnumber of oil droplets. The emulsion was knife coated using a 0.025setting of 2 ml hard aluminum foil previously coated with a 2 mil thickblack pigmented vinyl acetate based coating. The coating was dried bypassing high velocity room temperature air thereover until a surfaceskin was formed and then air dried overnight. By pouring the coating ona temporary carrier such as stainless steel or polytetrafluoroethylene,from which it is separable, a free film having magnetic inclusions canbe produced instead.

EXAMPLE 2

Color coded micromagnets were prepared with a binder of lacquercontaining appropriate color pigments. The lacquer was a widely marketedtype containing cellulose nitrate, ester gum, plasticizer, glycolesters, alcohols, aromatic and aliphatic hydrocarbons and was slightlythinned with lacquer thinner. By weight, a white portion contained 60parts of lacquer, and 50 parts of TiO.sub. 2 pigment. A red portioncontained 25 parts of red pigment, 10 parts barium ferrite, and 75 partsof lacquer. A small amount of corn starch was added to thicken theblends to a more easily spreadable viscosity.

The blends were coated in successive layers on a polyethylene carrierwith intermediate drying, a 1/2 mil thick white layer being appliedfirst followed by a 1/4 mil red layer. In spreading, the depth of eachlayer was controlled by drawing the sheet between spaced bars.

Several sheets of the coated carrier was stacked, each with the samecolor up, between the poles of a large electromagnet where they weresubjected to a strong magnetic field which magnetized the barium ferritecomponent. The sheets were then peeled from the carrier and broken upinto micromagnets capable of passing through a 325-mesh screen.

An oily suspending liquid was prepared having in parts by volume:

2 parts of light petroleum oil

1/4 part poppy seed oil

1/4 part paraffin oil

1/3 part fluorocarbon oil (Kel F Oil No. 1)

Into this blend was thoroughly mixed finely divided magnesium aluminumsilicate (Bentone), to increase the thixotropy and viscosity of the oil.Generally, it will be found desirable to increase the viscosity andthixotropy if the micromagnets to be used have a comparatively strongmagnetization, but to leave the liquid thinner and less viscous if themicromagnets are of a weaker magnetization.

Into this first suspending liquid was then thoroughly dispersed 1/6 partmicromagnets.

A second film former liquid was then prepared having in parts by volume:

1 part water

3/4 parts methanol (99.9% by volume)

3/4 parts ethanol (190 proof)

1/4 part glycerine

1/2 part gelatin

This mixture was heated to boil under constant stirring and poured intoa measured container. The liquid was allowed to cool about 120°F. and anamount of the first liquid equal to 1/2 of the volume of the second waspoured into the second liquid where, by reason of its heavier specificgravity, it sank to the bottom of the container. Under gentle agitationthe first liquid separated into tiny droplets, which emulsified in thegelatin liquid. Under continued agitation the droplets wereprogressively reduced in diameter. Agitation was discontinued when thesize of the oil droplets was slightly larger than the size of themicromagnets. The emulsion was then poured onto a plastic substrate toform a uniform coating. The substrate was opaque and black in color. Thegelatin liquid hardened with the micromagnet-containing oil inclusionsdistributed therethrough.

To increase the durability of the display device a protective coating oftransparent plastic such as clear liquid polyester resin which hardensto provide a durable surface can be applied over the gelatin coating.Alternatively, a transparent plastic may be laminated over the gelatincoating with a transparent adhesive.

The emulsion may be spread onto a transparent carrier such as a sheet ofMylar, which carrier can then be used as the viewing and markingsurface. A dark backing can be provided by adhering such as black clothto the film before the film has hardened.

A non-oil suspension carrier liquid may also be used. A low viscosityepoxy resin liquid (Celanese Epi-Rez 5077) having a specific gravity of1.13 - 1.15 and a viscosity of 500 - 500 cps. at 77° was mixed by volume21/2 parts to 3/4 parts with tetrachloromethane, having a specificgravity of 1.595, and into this mixture was thoroughly dispersed 1/2part colloidal silica powder thixotropic thickener (Cab-O-Sil). Nocuring agent was used with the resin, of course, since its function herewas to remain in a liquid state. The micromagnets were then suspended inthis and the liquid was emulsified in the gellatin film-former.

In the emulsification of microscopic droplets containing multi-coloredmicromagnets the problems faced are greater than those in theencapsulation of unmagnetized particles of powder. For instance, ifmicromagnets of a given magnetic strength are suspended in droplets of acarrier liquid having, inappropriately, a viscosity too weak andinsuficient to provide the restraint of them needed, they will, inattempting to join together magnetically, quickly work out of theirsuspension droplets to become embedded in the yet ungelled cellularwalls. In the foregoing examples the viscosity and thixotropy wasadjusted to prevent magnetic joining together of the micromagnets to adegree which would interfere with their enclosure in separate capsules.Bentonite, a thixotrophy producing suspending agent, was chosen as asuitable thickener, although other thixotrophy producing agents such asthe colloidal silica powder (Cab-O-Sil) are also useful for thispurpose. Optimum results are achieved with a thixotropic suspending orcarrier fluid.

If the suspension liquid is viscous and/or thixotropic enough toresemble a gel, little or no emphasis need be given to its density; butif it is fluid enough to readily flow then the degree of its viscosityand/or thixotropy, while adjusted to minimize unwanted magneticinteraction between the micromagnets, will also contribute towardholding the micromagnets in buoyant suspension and the degree of itsviscosity and/or thixotropy will determine to some extent the densityneeded to hold micromagnets of a given weight in suspension within theirdroplets at least a period of minutes (at least long enough to permitthe formation of a film from the film-former liquid), and preferablyindefinitely. If, in an easily flowing carrier liquid of a givenviscosity and/or thixotropy the density is inappropriately too heavy,the micromagnets will rise up out of their droplets or, if the densityis too light, they will sink down out of their droplets, in either casewith the hopeless projection of the micromagnets out of their suspensiondroplets to become embedded in the yet unhardened cellular walls. Theexemplary suspension liquids were therefore adjusted to have theappropriate density by the use of Kel-F Oil, a fluorocarbon oil having adensity of 1.9, which was mixed in suitable proportions with lighter oilto provide the workable density. Other suitable suspending liquids andblends of liquids prepared to have a chosen density will be apparent tothose skilled in the art.

In summary, any suitable suspension or carrier fluid for themicromagnets can be employed. The carrier fluid should be capable ofsurrounding the micromagnets so as to allow them to rotate when they areacted on by an exteriorily applied magnetic field. Its chemicalcomposition is immaterial provided the carrier fluid does not adverselyaffect the micromagnets or the polymer system. The carrier fluid shouldbe substantially insoluble in the system.

For optimum performance, the carrier fluid should have certaindensities, viscosities and thixotropies. Preferably, the density shouldbe approximately the same as that of the micromagnets so that they aresupported substantially at equilibrium without rising or sinking. Theviscosities and/or thixotropies should be such that the interaction ofthe micromagnets to each other and to an external magnetic field areproperly controlled. Thus, it is desirable to provide a carrier fluidwith viscosities and/or thixotropies so that a certain minimum forcemust be applied in order to rotate the magnet. Viscosities and/orthixotropies also provide a degree of stability to the display device byminimizing unwanted disorientations of the micromagnets. Densities,viscosities, thixotropies are imparted by the liquid itself, or mixturesof liquids, as well as by the introduction of density, viscosity and/orthixotropy agents.

There is an interrelation between density, viscosity and thixotropy inselecting the proper suspension or carrier fluid. The system isgenerally organic, preferably a hydrocarbon and/or a fatty oil.Glycerol, glycol, silicones and other fluids, as well as mixtures canalso be employed. Fluorocarbons, because of the high densities, are aconvenient method of increasing the density of fluid suspension system.Viscosity and/or thixotropies are imparted to the system by means ofgelling or thickening agents. Variation in the composition to achievethe desired function will be evident to one skilled in the art. Whereviscosities and/or thixotropies are properly selected densities and areof lesser importance. Thixotropic liquids, emulsions, gels, etc. can beused.

Thixotropic agents have the property, when dispersed in suitable media,of exhibiting a variable viscosity which depends on the shear stressapplied to the dispersion. At low shear stresses or at rest suchthixotropic dispersions have high viscosities in the nature of elasticsolids while at high shear stresses they have low viscosities.Thixotropic liquids are on-Newtonian whereas non-thixotropic liquids areNewtonian liquids, i.e., thixotropic liquids behave like elastic solidsat low shear or at rest and behave like liquids at high shear. Thus,they are fundamentally different from viscous non-thixotropic liquidswhich behave like liquids both at rest and under low and high shear.This phenonomen of thixotrophy is believed to be due to the formation ofa gel-like structure at low shear stresses with progressive disruptionas shear stresses increase. Typical thixotropic agents are collodialsilica such as Cab-O-Sil, etc., bentonite or kaolinite clays such aBentone, etc., carboxyl vinyl polymers such as those marketed by the B.F. Goodrich Chemical Company of Cleveland, Ohio, under the tradenames"Carbopol" and "Carboset", organometallic complexes such as thosemarketed by the Lubrizol Corporation of Cleveland, Ohio, under thetradename "Ircogel", Colloidal cellulose such as those marketed underthe Trademark "Avicel"; colloidal asbestos such as those marketed underthe Trademark "Avibest", by American Viscose Company, etc.

I have discovered that by controlling the thixotropy of the carrierfluid density and viscosity are of lesser importance since the selfadjustments of the thixotropic system imparts proper variableviscosities under stress and static conditions. Therefore, a thixotropiccarrier fluid is preferably employed in this invention to yield optimumresults. By employing thixotropic liquids (i.e. which behave like solidsat rest or low shear) the permanently magnetized micromagnets areprevented from magnetically interacting and clumping. Non-magnetizedparticles present no such problem since they do not interactmagnetically.

Stated another way, the thixotropic fluid encases the micromagnetics asfirmly and securely as a solid. Yet under a magnetic field whererotation is desired the area around the micromagnets forms islands ofliquids which allow rotation. Such properties are not imparted to thesystem by density or viscosity alone.

The relationship between the density of the carrier liquid dropletscontaining the micromagnets with th viscosity and the density of thefilm-forming composition is also important. It is desirable that thecarrier liquid droplets be held within the film-forming composition inbuoyant suspension until the film hardens and in the foregoing examplesthe density of the suspension liquid, carrying the micromagnets, waschosen to be slightly greater than the density of the film-formingcomposition. Unless the film-forming composition is quite viscous, themicromagnetic-containing liquid droplets, if lighter than thefilm-forming composition, will rise and break out through the surfacebefore the film has set.

After the emulsion or dispersion has been mixed and spread, the quickestpossible hardening time is preferred. The micromagnets and thesuspension droplets in which they are dispersed, having been agitated bythe mixing and the spreading, continue within the still wet emulsion tohave a considerable amount of motion and activity and while so doing thedroplets tend to meet and merge and the micromagnets tend to projecttheir surfaces out of the droplets to become adhered to the yet ungelledcellular walls.

In the first example of encapsulation the newly spread film wasimmediately subjected to high velocity air which speedily dried and setthe surface skin and started the hardening of the cellular walls. In thesecond example, rapidly evaporating alcohols in major proportion werepresent in the film-forming composition, and when the heated gelatinemulsion was poured into a thin coating to cool, it started to gel in amatter of seconds.

In the successful encapsulation of the micromagnets, mobile suspensionwithin microscopic inclusions, the relationship between the viscosityand thixotropy of the suspension liquid and the magnetic strength of themicromagnets is carefully chosen; the relationship between the densityand viscosity of the suspension liquid with the specific gravity of themicromagnets is also carefully chosen, the relationship between thedensity and viscosity of the suspension liquid with the density andviscosity of the film-forming polymer is also carefully chosen, and thematerials are selected to provide the quickest gel time after theemulsion has been coated onto its carrier.

Inclusions 24 are shown in the drawings to be spherical for purposes ofillustration, but in practice often assume a flattened shape on dryingof the coating. These flattened shapes can be made to overlie oneanother slightly to provide more complete coverage of the substrate.

FIGS. 4 and 5 show spherical or spheroidal multi-colored micromagnets 43in a liquid 45, sealed within prearranged geometric patterns onsubstrate 41 or substrate 42, by film-forming coatings 44 or 48.

FIG. 8 shows a greatly magnified side view of a spherical shapedmicromagnet, 43. For illustration color 46 is identified as blue andcolor 47 as white, though other contrasting colors may be used.Spherical or spheroidal multi-colored micromagnets were made of epoxyresin having a viscosity of 3,000 cps (Marblette Resin No. 658 withhardener No. 558, the Marblette Corporation). Mixed with 85 grams resinand 421/2 grams hardener, by weight, was 20 grams finely divided bariumferrite (EG-1 Powder, Stackpole Carbon Company), 40 grams blue powdertempera, and 5 grams titanium dioxide pigment. In a closed circularcontainer the composition, not yet hardened and still a liquid, wasemulsified, by brief, vigorous agitation, in 800 grams of mineral oil,SAE No. 40, which had been heated to 175°F., the oil in the emulsionbeing the continuous phase and the microscopic liquid epoxy spheresbeing the discontinuous phase. The container was mounted on a carriersupport which was motorized to move it first with a brief vigorousagitation, and then in an even circular pattern, this movement producinga steady centrifugal force which kept the hot emulsion constantly movingup and around the wall, thereby preventing the resin droplets fromsettling. The circle pattern had a diameter of about 4 inches and thespeed was about 125 rpm. The container had a capacity of about 2 litersand was bowl shaped in design, having a base diameter of 31/2 inches, aheight of 5 inches, and a lid diameter of 8 inches. A small air ventvalve, to relieve the pressure of the rapid air expansion caused by thesudden agitation of the hot oil inside the vessel, was provided in thelid which, otherwise, formed a tight seal on the vessel. Othercontainers may be used but the shape of the container contributes to thesuccess of the method. The hot resin droplets, while still liquid, mustbe prevented from touching one another in the suspension or they willadhere to each other and agglomorate and harden into a useless mass and,in the examples given herein the hot emulsion, under controlledcentrifugal force, moves easily from the small circumference to thelarge circumference, spreading its volume within the container andseparating farther the resin droplets until they are cured into hard,independent spheres.

After about 20 minutes of the described motion, the spheres hardened andthen were allowed to settle and the oil drained off. Oil residue wasremoved by cleaning with naphtha and the spheres were sized byscreening. Coloring was done by sprinkling the spheres in a single layeronto a paper carrier which had been coated with cellulose nitratelacquer greatly thinned and the spheres, adhering to the lacquer coatingon the paper carrier, were immersed about halfway into white vinyl inkwhich had been heavily pigmented with titanium dioxide pigment. Thecarrier was then passed between the poles on a magnet charger,directionally magnetizing the barium ferrite so that the white portionof the sphere identified one pole and the blue portion identified theother and when dry, the two colored spheres were brushed off, ready foruse.

In another instance, transparent spheres, each containing withinthemselves a platlet shaped color coded micromagnet, were made, therebyeliminating the painting requirement. Platlet shaped micromagnets ofabout 6 mils in size, were mixed, by volume about 10 parts wit 100 partslow viscosity polyester resin (Laminac Resin No. 4110, American CyanamidCo. with methyl ethyl ketone peroxide as the catalyst), and an oil-resinemulsion was formed by mixing the polyester with 800 grams mineral oilSAE No. 20 which had been heated to about 200° F., the oil being thecontinuous phase and the polyester spherical droplets, averaging about10 mils, being the discontinuous phase, and most of the polyesterdroplets having captured and enveloped within themselves a platletshaped clor coded micromagnet. As before described, the oily emulsionwas then kept in a circular motion until the polyester spheres hadhardened. Those carrying within them a micromagnet were then separatedfrom those that did not, by centrifuging, or by flotation in a mixtureof fluorocarbon oil and mineral oil, the proportionate ratios beingcarefully adjusted to provide a liquid with specific gravity in whichthe usable spheres, heavier because they carried within themselves amicromagnet, descended and separated from the lighter, all polyester,spheres which rose to the top.

In other instances, polyester was the hardenable material used for thepigmented micromagnets and clear epoxy was the material used for thetransparent spheres containing platlet shaped multicolored micromagnets.

The amounts in the examples above given were based on the use of thecircular container having about the capacity given, but does not limitthe process which may be enlarged for commercial productions.

In FIG. 4, the suspension liquid 45 was prepared with purified bentonite(Bentone 38 Nat. Lead Co.) mixed under high speed shear dispersion inclear mineral oil to provide a thixotropic gel which could be arranged,such as by extruding or molding, in fixed patterns on a substrate.Spherical micromagnets 43, of about 5 mils in diameter, were then mixedinto the gelled liquid and the mixture was extruded across a polyestersubstrate (Mylar) 41, in beads.

A rapid drying film-forming composition 44, prepared of, by weight, 10parts cellulose nitrate (RS 125/175 sec., Hercules, Inc.) dissolved in150 parts acetone, 5 parts isophorone, and 6 parts saturated polyesterplasticizer (Harflex 335, Wallace & Tiernan, Inc.), was coated to coverso as to leave after the solvent had evaporated, a film having athickness of about 1 mil -- thus sealing the thixotropic gel within thepre-arranged pattern. Next, the vacant spaces between the beads werefilled with the micromagnet-liquid mixture and the film-formingcomposition was coated to cover again. When this film was dry andhardened, the micromagnets within the sheet, were now positivelycontained between transparent structural partitions.

The cellular side may be given further protection and structuralsupport. The cellular surface is usually somewhat uneven and acatalyst-hardening composition such as epoxy or polyester, coated overthe cells, fills the uneven contours and hardens into a smooth outsidesurface. This optional coating is identified in FIG. 4 as 40.

The relationship of the resin, resin solvent, and plasticizer of thefilm-forming composition and the suspension or carrier fluid gel whichit is to enclose are important. The resin solvent used wassatisfactorily immiscible with the carrier liquid being enclosed. Theratio of solvent to resin provided a viscosity which could beconveniently spread. And, since the film is expected to dry and hardenaround the contour of a deposit of carrier liquid, certain criticallimits exist. Improperly plasticized, the film may, as the solventleaves, tighten and violently draw the carrier fluid gel deposit into anunusable distortion. And if the film does not sufficiently stretch, itmay, in drying, rupture, forcing out some of the carrier fluid gel as itdries and tightens.

Other suspension liquids for the micromagnets may also be used. Glycerolmixed with Cab-O-Sil to a usable carrier fluid gel was arranged on acellulose acetate substrate in a broken pattern. The deposits of theglycerol micromagnets mixture were then covered with a film-formingcomposition consisting of 10 parts polyacrylic resin (n-Butylmethacrylate DuPont Elvacite No. 2044) and 2 parts polyacrylic resin(Isobutyl methacrylate DuPont Elvacite No. 2045) dissolved in 100 partstrichloroethylene. After the film had formed and dried, sealing into thepre-arranged cells the glycerol-micromagnet mixture, the vacant spaceswere filled and the second coating was applied to cover.

A suspension liquid of, by volume, 50 parts glycerol and 50 partsethylene glycol, into which was mixed Cab-O-Sil was also used. Theethylene glycol was used to reduce the viscosity of the suspensionliquid and a relatively greater amount of Cab-O-Sil thickener was thenused to increase the thixotropy. It has been found that themicro-magnets respond more instantly in a liquid in which the suspensionqualities have been provided with emphasis on a thixotropic structurerather than a heavy viscosity. Thus, thixotrophy, as distinguished fromviscosity alone in the suspension liquid yields a more desirable andresponsive device.

This suspension liquid, thickened to more or less a moldable carrierfluid gel and containing the micromagnets, was then arranged in a brokenpattern on a Mylar substrate and coated with the above acrylic filmformer as described.

The broken pattern of the micromagnet-liquid mixture on the substratemay be arranged in several possible geometric patterns. Parallel beadsand grooves have been described but checkerboard spacing of such shapesas squares, triangles, rectangles, and circles are also useful.

Also, a broken pattern can be arranged which requires only one coatingof the film-forming composition. Beads of the micromagnet gel mixture,each bead extruded to have the cross sectional shape of a parallelogram,and the beads extruded quite close together so that they do not touchbut slightly overlap, can then be covered with the film-formingcomposition which, while covering, fills the separating spaces betweenand then hardens.

FIG. 5 shows transparent carrier sheet, 48, formed to provide, on eitherside, depressions in which the micromagnet mixture may be deposited.Film-forming composition 42 was then applied to cover both sides,sealing, in permanent geographic locations within the structure, themicromagnet mixture. The fluted contours of transparent carrier 48 wasby choice designed to provide a slight overlapping of the micromagentmixture to give the effect of an unbroken appearance, but alsosatisfactory is a flat transparent sheet on which the micromagnetmixture is deposited in broken patterns on one side which slightlyoverlap the broken patterns on the other. Added and adhered to eitherside of the structure may then be protective sheets and backing.

A transparent film forms a useful dividing partition but since it mayrefract incident light, its presence may be slightly discernible. It hasbeen found that a film disrupted in its continuity by such as minuteindentations or holes, is optically superior. A one mil clear film, forexample, thoroughly perforated with holes, preferably smaller that themicromagnets used, becomes virtually invisible within the structure.Also the same film, used as a substrate for broken pattern deposits,permits the suspension liquid to fill the holes within it and minimizesany visible lines of distinction between the deposits. Such a netlikesubstance, carrying the micromagnet deposits can be sealed on theexposed side by a suitable adhesive sealant and/or enclosed within anenvelope structure to have at least one transparent side.

Drying of the film-forming compositions on a sheet about 10 inchessquare was speeded by heating for 4 minutes, 12 inches under a 250 wattinfra-red bulb. Coating was done by spraying or pouring.

The substrate carrying on it the micromagnet liquid mixture in cellsprovides a device which can be used to provide a visual display. If thesubstrate is transparent it may be used as the viewing surface. If it isopaque, the transparent cellular side would then be used for viewing.

FIGS. 6 and 7 show pre-constructed carriers for the micromagnet mixture.In FIG. 6 transparent corrugated divider 52 separates sheet 50 fromsheet 51, at least one sheet of which is transparent, and forms withsaid sheets slightly overlapping triangular shaped compartments for themicromagnet mixture. Divider 52 was made of cellulose acetate 0.001 filmcorrugated between heated right and left threaded 1 in. rollers havingUNS-10 threads. A solvent of, by volume, 4 parts acetone and 1 partisophorone was first lightly spread by roller onto sheets 50 and 51which were 0.010 cellulose acetate, and they were then adhered to thecorrugated divider as shown. Placed between two panes of glass andheated four minutes at 200° F., the union become permanent and warpingof the structure was prevented.

Micromagnets averaging about 6 mils were then mixed with clear mineraloil that had been thickened with Cab-O-Sil and the mixture was drawn into fill the triangular flutes by suction applied to one end. The endswere then sealed with cellulose acetate which had been dissolved, byweight, 1 part to 10 parts acetone.

FIG. 7 shows a carrier internally partitioned by individual transparentwalls 53, inclined to provide a slight overlapping. Contained betweenthe partitions are the micromagnets 43 in the suspension liquid 45 andother partitioning spacers will be apparent to those skilled in the art.

In the pre-constructed carriers, the suspension liquid may be such asoil, glycerine, or silicone fluid, or any liquid that does notchemically attack the material of the carrier or the micromagnets.

It is usually helpful, with any of the carrier devices disclosed herein,to use a backing having a dark color, which will absorb the incidentlight falling between the micromagnets. A light colored background,reflecting the incident light tends to interfere with and diminish theresolution of the visual display.

In selecting the substrate or backing, the purpose for which the deviceis intended is first considered. If the external activating magneticforce is to be applied from behind, as with a read-out indicator, thesubstrate would be chosen from a nonmagnetic material, such as plasticor paper.

In the case of a marking device, one in which the activating magneticforce is applied from the front, as with a magnetized writing stylus, amagnetizable back plate, such as a soft iron sheet, for example, willprovide a conductor which will tend to concentrate the magnetic lines offorce from the writing stylus, thus narrowing and strengthening itsinfluence on the micromagnets and sharpening the resolution of thepattern they form. A carrier, such as paperboard, or a plastic sheet,coated on one surface with a suitable adhesive and then covered withiron filings, or other powdered magnetic material, will also provide asatisfactory magnetic backing.

In all of the foregoing examples micromagents having identical colorcodings were used. It is possible, however, to mix various colorstogether and obtain a new color result. A sheet containing, for example,yellow and white micromagents, mixed with others of blue and white, willregister a display of green and white. The use of such combinations canbe advantageous in minimizing the number of micromagnets which need beprepared to form a wide variety of different colored display devices.

Materials hardened by catalyst action may be used as well asfilm-forming materials hardened by the evaporation of solvents. Beads ofmicromagnet-oil mixture were patterned across a 1 mil polyestersubstrate and covered with crystal clear epoxy resin (Marblette ResinNo. 658 with hardener No. 558, the Marblette Corporation). A smoothsheet of polyester material (Mylar) was then placed across the epoxy andafter the epoxy had hardened, the polyester sheet was peeled off leavinga smooth outer surface. The procedure was then repeated on the reverseside of the substrate, except that the beads of micromagnet-oil mixturewere alternated, geographically, with those on the other side, therebymaking the board appear to carry the micromagents throughout in evendispersion. Since the epoxy would not adhere permanently to a polyestersheet, the substrate was first thoroughly perforated with minute holesand then coated thinly coated with the epoxy. Here, the perforationspermitted the epoxy coating on both sides to join thru the substrate toform a permanent bond. The carrier liquid for the micromagnets consistedof mineral oil thickened with Bentone.

A wide variety of materials useful as film-formers can be employed inpreparing the device depending upon the particular part of the devicefor which it is employed, the method of preparation, etc. For example,certain materials are preferred in preparing the exterior films ofsheets, other materials are preferred for the interior compartments,while other materials are preferred in preparing the included cells, andstill other materials are preferred for coating areas offluid-surrounded micromagnets to build up sequential compartments.Examples of each have been presented herein. These materials aregenerally polymers, particularly film-forming polymers, both organic andinorganic, including both natural and synthetic polymers or mixtures ofboth natural and synthetic polymers.

The essence of the present invention relates to the use of compartmentsor cells to prevent displacement of the multi-colored micromagnets fromtheir assigned position in the sheet without distracting from thedesired function of the micromagnets such as rotability. By preventingdisplacement, the desired distribution of the micromagnets is maintainedeven under rigorous use. Materials and techniques for achieving thishave been described herein.

As is quite evident other film formers and polymers, other suspending orcarrier fluids permitting mobility including thixotropic agents, othermicromagnets, etc. are known or will be constantly developed which couldbe useful in this invention. It is, therefore, not only impossible toattempt a comprehensive catalogue of such components, but to attempt todescribe the invention in its broader aspects in terms of specificchemical names of all components that could be used would be toovoluminous and unnecessary since one skilled in the art could byfollowing the description of the invention herein select usefulpolymers, fluids and micromagnetics. This invention lies in thepreparation of suitable multi-colored, magnetically activated displaydevices and their individual components are important only in the sensethat they affect the preparation of a suitable device. To preciselydefine each possible component and each possible variation inpreparative techniques in light of the present disclosure would merelycall for knowledge within the skill of the art in a manner analogous toa mechanical engineer who prescribes in the construction of a machinethe proper materials and the proper dimensions thereof. From thedescription in this specification and with the knowledge of one skilledin the art, one will know or deduce with confidence the applicability ofspecific components suitable in this invention. In analogy to the caseof a machine, wherein the use of certain materials of construction ordimensions of parts would lead to no practical or useful result, variousmaterials will be rejected as inapplicable while others would beoperative. One can obviously assume that no one will wish to make auseless multi-colored magnetically operated display device nor will bemisled because it is possible to misapply the teachings of the presentdisclosure to do so.

Thus, the examples given herein are intended to be illustrative andvarious modifications and changes in the materials and structures may beapparent to those skilled in the art without departing from the spiritof this invention.

For example, the oil surrounding the micromagnets for the most partseparates them from the epoxy composition sufficiently to preventadhesive bonding but while the epoxy system is curing it maynevertheless touch and tend to adhere to some of the micromagnets andsuch unwanted bonding was minimized or prevented during the curingperiod by occasionally rotating the micromagnets by an externallyapplied activating magnetic field.

In another embodiment the micro-magnet oil mixture was positioned in abroken pattern on a 0.025 sheet of aluminum substrate and covered with acoating of catalyst hardenable polyester (Laminac resin No. 4110,American Cyanamid Company with methyl ethyl ketone peroxide as thecatalyst) which was then allowed to harden.

The micromagnet-oil mixture was then patterned across to alternategeographically with the first deposits and a second coating of thepolyester resin and then applied to cover and allowed to harden. Eachcoating, while curing, showed a tendency to "crawl" and it was foundthat this untoward movement could be virtually eliminated by spreadingacross the freshly poured coating a restraining web such as a thindiaphanous like sheet of tissue paper which, when saturated with thepolyester resin, became transparent and virtually undetectable withinthe device. During the curing periods the micromagnets were occasionallymagnetically rotated to discourage the formation of any adhesive bondbetween them and the polyester resin. After the second coating hadhardened the board was useable for purposes of providing a visualdisplay. A third coating, however, was then spread and across it wasplaced a smooth sheet of Mylar, this sheet being unrolled across thecoating in a manner which pushed ahead and out any air bubbles. Afterhardening, the Mylar sheet was peeled off and the third coating thencontributed a smooth surface as well as further structural strengtheningto the device. As is well known the curing period of most catalysthardening systems can be speeded by the application of heat.

The catalyst hardenable compositions as described provided a rigidlyconstructed durable device capable of withstanding considerable physicalhandling or abuse such as might be expected by a device adapted for use,for example, as a marking board for children.

I claim:
 1. In a device adapted to provide a visual display when anexterior magnetic force is applied thereto, said device comprising aplurality of rotatable, multi-colored, non-magnetically interactive,permanently magnetized micromagnets disposed in a cavity within a bodyhaving a transparent surface; the improvement that comprises thepresence of a plurality of relatively small fluid-containingcompartments, in which said micromagnets are contained, within saidcavity; said device being a sheet containing the micromagnets in arelatively thin cavity between the front and rear surfaces of the sheet;with the proviso that the micromagnets in the sheet are sealed intocells or compartments by coating areas of rotatable micromagnets intocells or compartments by coating areas of rotatable micromagnets with ahardenable film-former so as to form a sheet having rotatablemicromagnet inclusions in the sheet.
 2. A method of forming a sheet ofclaim 1 adapted to provide a visual display when an exterior magneticfield is applied thereto which comprises:1. depositing relatively smallareas of carrier fluid-surrounded, non-magnetically interactive,permanently magnetized, multi-colored micromagnets on a surface; 2.coating said small areas with a hardenable film-former; and
 3. hardeningsaid film-former into sealed compartments of small areas offluid-surrounded, non-magnetically interactive, permanently magnetized,multi-colored micromagnets.
 3. The device of claim 1 where a thixotropiccarrier fluid surrounds the micromagnets.
 4. The method of claim 2 wherethe carrier fluid is thixotropic.